Recording materials determine the daily routine of businesses and professional life. Various solutions for authenticating security features have already been suggested in the past for use with recording materials whose authorized use must generally be verified by a specific mark on the material permitted for this purpose.
Proof of authenticity for documents in the form of passive inspection is made possible, for example, through watermarks. By watermark is meant, generally, a mark in the paper which is generated through different paper thicknesses. A distinction is made between true watermarks which are produced by displacement (so-called light watermarks) or by concentration (so-called shaded watermark) of the fiber pulp using, for example, a dandy roll in the wire section of a paper machine, impressed watermarks, also called Molette watermarks, which are produced by impressing the paper while still wet in the press section of a paper machine, and, finally, imitation watermarks which are made either by imprinting the finished paper outside the paper machine with a colorless varnish or by stamping the finished paper outside the paper machine.
Suggestions for imitation watermarks are found in EP 0 203 499 B1, which provides a paper web which, through the addition of suitable substances, is deformable by means of heat and possibly also under the simultaneous application of pressure, and in DE 39 20 378 A1 which discloses the arrangement of a watermark outside the paper machine by imprinting with a varnish. The disadvantages of these suggestions consist in that illegal copying of a watermark of this kind is rather easy.
According to a suggestion in DE 690 01 677 T2, a synthetic print medium with pseudo-watermarks is provided. This known print medium comprises a substrate of plastic, at least one authentication mark or security mark which is preferably applied by intaglio printing and which changes the opacity of the print medium, and at least one printable pigment coating covering the mark. The monochromatic or polychromatic mark is barely visible in reflected light but can easily be seen under transmitted light. This known print medium also has the basic drawback that the imprinted pseudo-watermark can be forged relatively easily, which can also not be prevented by simple pigment coatings applied thereto.
Heat-sensitive recording materials with authenticating security features in the form of watermarks are also generally known. For example, EP 0 844 097 A1 discloses, as a first security feature for a heat-sensitive recording material in the form of an imitation watermark, a latent image which is printed on the back side of the recording material and is produced by a security ink containing a fluorescent reagent. The security ink contains a water-repelling agent to form a second security feature in the form of a waterproof image on the back side of the heat-sensitive recording material. The security ink containing the fluorescent reagent, which is used as pigment or dye, and the water-repelling agent is contained or dispersed in an aqueous carrier which, in addition to these components, can also contain a binder.
As is generally the case in all of the suggestions relying on the use of imitation watermarks, the disadvantage in this suggestion consists in that counterfeiting or copying can be accomplished fairly easily. Even if the desired security features, or those to be formed, are carried out in the midst of the production process, additional problems arise with respect to high investment costs and sometimes also poor flexibility.